1. Field of the Invention
This invention relates to methods of and apparatus for the generation of radio frequency signals, and, in particular, to those utilizing a double-external-cavity diode laser for the generation of extremely high frequency electrical signals. According, it is a general object of this invention to provide new and improved methods therefore.
2. General Background
Both time domain and frequency domain techniques for characterizing the frequency response of high speed photodetectors have been discussed in the literature.
The time domain method utilizes the illumination of the detector surface with ultrashort light pulses from mode-locked or q-switched lasers, as reported by J. E. Bowers, C. A. Burrus, and R. J. McCoy, "InGaAs PIN Detectors with Modulation Response to Millimeter Wavelengths", Electron. Lett. 21 (18), pp. 812-814 (1985), and by P. W. Smith and Y. Silverberg, "Passive Mode-Locking and Short Pulse Formation in Semiconductor Diode Lasers", CLEO'85 Technical Digest, paper THC3 (1985), and references therein. Disadvantageously, the application of time-domain techniques requires the use of complicated and expensive laser apparatus that are inconvenient to set up and operate.
The frequency domain methods require either the illumination of the photodetector with the modulated output of a high speed laser or the detection of the beat frequency (difference in optical frequency) between two signals which are combined at the detector surface. Disadvantageously, the use of high speed diode lasers to measure detector response has been limited to electrical bandwidths of less than 20 GHz, and the results were often difficult to interpret, due to uncertainty about the laser modulation characteristics. While the beat frequency technique could be used to generate difference frequencies of hundreds of gigahertz, the realization of this approach, heretofore, has required either the combination of the optical output of two external-cavity single-mode lasers with narrow linewidths, as reported by K. H. Cameron, M. R. Matthews, T. G. Hodgkinson, and W. J. Devlin, "Frequency-Stable Packaged 20-kHz Linewidth 1.5 .mu.m InGaAsP External Cavity Laser Used in an Experimental Heterodyne Optical Fiber System", CLEO'85 Technical Digest, paper TUC5 (1985), and references therein, or the utilization of a frequency modulated (FM) sideband injection locking technique with two diode lasers in a master-slave configuration, as reported by L. Goldberg, A. M. Yurek, H. F. Taylor, and J. F. Weller, "35 GHz Microwave Signal Generation with an Injection-Locked Laser Diode", Electron. Lett. 21 (18), pp. 814-815 (1985).